In the field of copper refining, various methods have been proposed for recovering copper from an object to be treated containing copper (hereinafter, referred to as a “copper-bearing material”) such as a copper ore or a copper concentrate. For example, in order to recover copper from a copper sulfide ore which is one form of a copper-bearing material, the copper sulfide ore is generally treated by the following steps.
(1) Flotation Step
In the flotation step, a copper ore obtained from a mine is ground and then mixed with water to prepare a slurry, and the slurry is subjected to flotation. The flotation is performed by adding a flotation agent containing a depressant, a frother, and a collector to the slurry and by blowing air into the slurry. As a result, a copper-bearing mineral is separated as a float fraction and gangue is separated as a sink fraction. In this way, a copper concentrate with a copper grade of about 30% is obtained. The obtained copper concentrate is sent to the next pyrometallurgical smelting step.
(2) Pyrometallurgical Smelting Step
In the pyrometallurgical smelting step, the copper concentrate obtained in the above flotation step is smelted using a furnace such as a flash furnace, and then refined in a converter and an anode furnace to obtain blister copper with a copper grade of about 99%. The blister copper is cast into anodes and then sent to the next electrolysis step. This pyrometallurgical smelting process distributes arsenic contained in the copper concentrate among slag, dust, and the blister copper. The slag is granulated with water and used as, for example, a land-fill material. The dust is returned to the furnace. Sulfur contained in the copper concentrate is separated as sulfur dioxide gas and used as a raw material of sulfuric acid.
(3) Electrolysis Step
In the electrolysis step, the anodes are placed in an electrolytic cell filled with a sulfuric acidic solution (electrolytic solution) and electric current is passed between the anodes and cathodes to perform electrolytic refining. As a result, copper is dissolved from the anodes and deposited on the cathodes as electrolytic copper which is a product with a purity of 99.99%. Along with the electrolysis step, the arsenic distributed to the anodes is eluted into the electrolytic solution. The eluted arsenic is recovered as decopperized slime by decopperizing electrolysis. The decopperized slime is used as an intermediate material or returned to the furnace.
The arsenic distributed to the slag in the pyrometallurgical smelting step is fixed in a stable form in the slag. On the other hand, the arsenic distributed to the dust and the decopperized slime is in an unstable form, and therefore it is not preferable that the dust and the decopperized slime are directly discharged to the outside of the system and disposed of. For this reason, the dust and the decopperized slime are returned to the furnace or further treated through an additional process. In this way, most of the arsenic contained in the copper concentrate is finally distributed to slag and fixed in a stable form in the slag.
Recently, the situation with regard to raw materials of copper has been changed. More specifically, the impurity grade, especially arsenic grade of copper ores is increasing year after year, and therefore the arsenic grade of copper concentrates obtained from the copper ores is also increasing gradually. For example, the arsenic grade of copper concentrates is conventionally about 0.1 to 0.2%, but in recent years, it is not unusual that the arsenic grade of copper concentrates exceeds 1%. Due to such an increase in the arsenic content of copper concentrates, there is a case where existing slag treatment equipment cannot cope with an increase in the amount of arsenic fixed in slag in spite of the fact that the amount of copper concentrate treated is the same as before. Such a problem can be solved by, for example, providing new slag treatment equipment or increasing the capacity of the existing slag treatment equipment, but this requires a significant investment and therefore increases cost.
If the arsenic grade of a copper concentrate obtained from a copper ore with high arsenic grade can be reduced to, for example, the same level as before by separating and removing arsenic from the copper ore with high arsenic grade, the load of arsenic to be treated could be kept at the same level as before, which eliminates the necessity of making such a capital investment.
In this regard, Patent Document 1 proposes a method for separating arsenopyrite contained in iron pyrite by flotation. According to this method, a sulfuric acid-based depressant containing hydrogen sulfite ions, such as sodium hydrogen sulfite, is added to iron pyrite to prepare a slurry, and then the slurry is subjected to flotation under conditions where the pH of the slurry is maintained at 8 or less and the temperature of the slurry is 30° C. or higher to separate arsenopyrite from the iron pyrite.
However, it is difficult to directly apply this method to the separation of arsenic from a copper ore or copper concentrate. This is because, in most cases, arsenic is present as an arsenic mineral such as tennantite ((CuFe)12As4S13) or enargite (Cu3AsS4) in, for example, a copper concentrate mainly containing chalcopyrite or bornite and these arsenic minerals have floatability similar to that of chalcopyrite or bornite, which makes it difficult to separate copper and arsenic from each other by flotation.
Further, Patent Document 2 proposes a method for separating an arsenic mineral contained in an arsenic-bearing copper concentrate. According to this method, a copper concentrate is thermally treated at 90 to 120° C., and then potassium hexacyanoferrate (II) (yellow prussiate of potash: K4[Fe(CN)6]) as a depressant for depressing copper is added in an amount of 10 to 15 kg per ton of copper concentrate so that an arsenic mineral is separated as a float fraction and chalcopyrite or bornite is separated as a sink fraction.
This method oxidizes a surface of the copper mineral in a copper concentrate by heating so as to form inactive oxide film on the surface, which is believed to cause a difference in surface conditions between the copper-mineral and the arsenic mineral from the viewpoint of surface chemistry or crystal chemistry. However, the practical use of this method requires equipment and energy for heating a large amount of copper concentrate, which leads to an increase in cost.
Further, Non-Patent Document 1 proposes a flotation method in which a slurry containing copper minerals is treated with hydrogen peroxide and then sodium nitrate is added to adjust the pH of the slurry to 5. Non-Patent Document 1 also proposes a flotation method in which hydrogen peroxide and EDTA are added to copper minerals, and then potassium hydroxide is added to adjust pH to 11. However, these two methods use deleterious substances and therefore have safety problems associated with handling of these deleterious substances as well as cost problems.
As has been described above, it is difficult for any of these conventional methods to efficiently separate an arsenic mineral from a copper-bearing material by flotation.